This time last year the 19-year-old had yet to play in the Premier League and was turning out for MK Dons in League Two.
At the weekend, he was man of the match in England’s 3-2 win over world champions Germany.
“There is always a question mark with a young player,” said Pochettino.
“You can assess his left and right foot, his performance on the pitch and in training.
“But how a young player can be affected for the big things that happen in football, being a senior player, being in the national team, this is always a question mark.
“You can be aware and behind him to try to support him. But I think that in the same it has surprised us how he has shown maturity.
“He is very calm after all that happened to him and that has changed his life in the same way that when he arrived one year ago.”